BELGRADE, Yugoslavia (AP) _ The Chinese Embassy was set ablaze after NATO warplanes pounded Belgrade early Saturday in the heaviest night of attacks on the capital so far. Hours earlier, allied cluster bombs ripped through civilian areas in another city, reportedly killing at least 15 people.

The Yugoslav army headquarters and other government buildings were also attacked in intense air raids that plunged much of the city into darkness.

Two people were killed, more than 20 injured and two were missing, China's official Xinhua News Agency said, adding one of its reporters was among the dead. The agency said three NATO missiles slammed into the embassy.

``The People's Republic (of China) has been attacked,'' Chinese Ambassador Pan Zhanlien said as he stood before the shattered embassy in the capital's New Belgrade district. Fire trucks converged on the building, which was engulfed in smoke.

The bombing could complicate efforts to reach a peace agreement in Yugoslavia. China is a permanent member of the U.N. Security Council, which NATO is trying to get to approve its peace plan. China had been strongly opposed to the bombing campaign and insisted the Kosovo crisis must be solved by diplomatic means.

NATO said it did not intentionally target the Chinese Embassy. In a statement from its headquarters in Brussels, Belgium, it said it ``regrets any damage to the embassy or injuries to Chinese diplomatic personnel.''

The American ambassador to the United Nations, Peter Burleigh, said he had spoken to his Chinese counterpart and told him that ``in case it was a NATO bomb, it is obviously something we deeply regret and we apologize for.''

Burleigh spoke as he headed into an emergency Security Council session over the embassy attack, which began about 12:45 a.m. Saturday.

The bombing could also anger Russia which had called on NATO to halt the bombing to give diplomacy a chance, and only reluctantly agreed to support the peace plan.

In New York, U.N. chief Kofi Annan said he was shocked by the attacks in both Belgrade and Nis. He was ``distressed to learn that NATO airstrikes apparently hit civilian buildings in Yugoslavia on Friday ... with attendant loss of life and many injured,'' Annan's spokesman Fred Eckhard said.

``The Chinese government and people express their utmost indignation and severe condemnation of the barbarian act,'' China said in a statement. ``U.S.-led NATO should bear all responsibilities.''

The statement, reported by Xinhua and broadcast on nationwide television, said the government reserved the right to take further action over the embassy bombing, but gave no details.

Pentagon spokesman Kenneth Bacon would not confirm that a NATO missile went awry. However, he said NATO was targeting at least one building across the street from the embassy.

Outside the embassy, stunned and bloodied members of the 30-person staff, who live in the building, huddled with each other as firefighters and emergency personnel converged on the area.

A Chinese diplomat appeared on Serbian television, his clothes and hands soaked in blood. ``We haven't found some of my colleagues,'' he said, identifying himself only as an adviser on cultural affairs.

Firemen in oxygen tanks made their way through the choking smoke in search of injured. The Xinhua agency said one of the two dead was one of its journalists.

Two gaping holes were visible in the building, and a six-foot-deep crater was blown out at one corner. One side of the building was heavily damaged, with rubble and broken concrete strewn all around.

NATO said it conducted its ``most concentrated'' attack to date. The entire city was thrown into darkness by a blackout soon after nightfall Friday. Waves of strong detonations continued to rock the capital through the early morning hours along with bursts of anti-aircraft fire. Podgorica, the capital of the smaller Yugoslav republic of Montenegro, also lost power. The all-clear was sounded in Belgrade around dawn Saturday.

The state-run Tanjug news agency said NATO had used graphite bombs to cripple the power system late Friday. The so-called ``soft-bombs,'' which NATO confirmed using in strikes on power stations Sunday, explode over the target, dispersing strips of graphite which land on sensitive electrical equipment, causing it to short-circuit.

Witnesses also reported a huge fire in the direction of the capital's main Obrenovac power plant.

The Hotel Yugoslavia, near the Chinese Embassy, also was struck, causing a huge fire. NATO said it targeted the hotel because it served as a barracks for Serb special police forces. In downtown Belgrade, the Yugoslav Army headquarters, the Defense Ministry, General Staff, a large police precinct and the Interior Ministry also were struck, Yugoslavia media reported.

Almost all the windows along Kneza Milosa street, a main Belgrade thoroughfare where many embassies are located, were shattered for hundreds of yards. Debris, rubble, office equipment, burning papers were strewn all over the streets.

Late Friday night, the alliance also acknowledged that it was using cluster bombs in an attack on a military airport in Nis and that it was ``highly probable'' one bomb went astray and hit a residential neighborhood.

Earlier, journalists escorted by Serbian authorities were taken to Nis, an industrial city 120 miles southeast of Belgrade, to view the devastation from the cluster bomb attack, which hit a market and a hospital complex.

Buildings were pockmarked from the bombs, which release explosives that then explode again. More bombs lay unexploded in the streets.

At the market, where nine people were killed, the body of one elderly woman was partially dismembered. Just a few steps away, blood trickled from the body of a young man.

A statement from NATO headquarters in Brussels, Belgium, said the target of the attack had been Nis airfield. ``Unfortunately, it is highly probable that a weapon went astray and hit civilian buildings,'' it said. ``There was no intent to harm civilians.''

The latest attacks came after foreign ministers from Russia and the major Western powers on Thursday agreed on a draft plan for ending the conflict. The plan includes the deployment of ``effective international civil and security presences'' in Kosovo, a province of Yugoslavia's main republic, Serbia. President Slobodan Milosevic's government signaled it would eventually accept the deal _ if the alliance stops the bombing.

Tanjug said the agreement was ``only the start of a very lengthy and complex process, which will probably end successfully.'' It reiterated that NATO must stop bombing Yugoslav targets before the plan is negotiated.

Many details must be worked out. Russia wants the bombing to stop, while NATO insists the Serbs first stop the repression of ethnic Albanians. The United States wants a complete pullout of Serb forces, while Moscow and Belgrade talk of a partial pullout.

NATO launched the air attacks March 24 to force Milosevic into compliance. Nearly 700,000 ethnic Albanians have fled the province since then.

Ethnic Albanian rebels said Friday the situation in Kosovo was deteriorating, with people in the central Drenica region facing starvation.